Method for adapting charge initiation for an implantable cardioverter-defibrillator

ABSTRACT

Adaptive methods for initiating charging of the high power capacitors of an implantable medical device for therapy delivery after the patient experiences a non-sustained arrhythmia. The adaptive methods adjust persistence criteria used to analyze an arrhythmia prior to initiating a charging sequence to deliver therapy.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/042,911, filed Jan. 25, 2005 and titled METHOD FOR ADAPTING CHARGEINITIATION FOR AN IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR, the entiredisclosure of which is herein incorporated by reference.

The present application is related to U.S. patent application Ser. No.11/043,012, filed Jan. 25, 2005, published as US Patent ApplicationPublication Number 2006-0167504 and titled DEVICES FOR ADAPTING CHARGEINITIATION FOR AN IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR.

FIELD

The present invention relates generally to implantable cardiac systemsthat detect, sense and classify cardiac signals. More particularly, thepresent invention relates to implantable medical devices that can adaptthe initiation of a therapeutic therapy for patients who experiencerecurring non-sustained arrhythmic episodes.

BACKGROUND

Ventricular tachycardia (VT) is a difficult clinical problem for thephysician. Its evaluation and treatment are complicated because it oftenoccurs in life-threatening situations that dictate rapid diagnosis andtreatment. VT is defined as three or more beats of ventricular origin insuccession at a rate greater than 100 beats/minute. The rhythm isfrequently regular, but on occasion it may be modestly to whollyirregular. The arrhythmia may be either well-tolerated or associatedwith grave, life-threatening hemodynamic compromise. The hemodynamicconsequences of VT depend largely on the presence or absence ormyocardial dysfunction (such as might result from ischemia orinfarction) and on the rate of VT.

VT can be referred to as sustained or nonsustained. Sustained VT refersto an episode that lasts at least 30 seconds and generally requirestermination by antiarrhythmic drugs, antitachycardia pacing techniquesor electrical cardioversion. Nonsustained VT refers to episodes that arelonger than three beats but terminate spontaneously generally within 30seconds.

Implantable cardiac rhythm management devices are an effective treatmentin managing irregular cardiac rhythms in particular patients.Implantable cardiac rhythm management devices are capable of recognizingand treating arrhythmias with a variety of therapies. For the reasonsstated above, and for other reasons stated below, which will becomeapparent to those skilled in the art upon reading and understanding thepresent specification, there is a need in the art for providing a methodfor adapting the initiation of a therapeutic therapy for those patientswho experience recurring non-sustained arrhythmic episodes.

SUMMARY

The present invention, in an illustrative embodiment, includes a methodof cardiac treatment using an implanted medical device system havingimplanted electrodes and an energy storage system for storing electriccharge for use in cardiac stimulus, the method comprising determiningthat cardiac stimulus is indicated at a first time, initiating acharging operation for the energy storage system, determining whetherthe cardiac stimulus is no longer indicated at a second time after thefirst time but prior to delivery of the indicated cardiac stimulus, and,if the cardiac stimulus is no longer indicated, changing a thresholdused to determine whether cardiac stimulus is indicated. The second timemay occur prior to completion of the charging operation, and/or aftercompletion of the charging operation. The determining step may beperformed repeatedly during the charging operation. If the cardiacstimulus continues to be indicated during the charging operation, thedetermining step may be performed at least once after the chargingoperation is completed. If the determining step fails, the method mayfurther include delivering cardiac stimulus.

Another illustrative embodiment includes a method of cardiac treatmentusing an implantable medical device system having implanted electrodesand an energy storage system for storing charge for performing cardiacelectrical stimulus, the method comprising capturing a predeterminednumber of selected cardiac events using the implanted electrodes,determining whether a threshold proportion of the predetermined numberof selected cardiac events are abnormal, and, if treatment is indicated:initiating a charging operation for the energy storage system, anddetermining whether cardiac rhythm has returned to normal and, if so,increasing the threshold proportion. The threshold proportion may be, atleast initially, in the range of 70-80%. The step of increasing thethreshold proportion may include increasing the predetermined number.The illustrative method may further comprise determining if treatment isindicated if: at least the threshold proportion of the predeterminednumber of selected events are abnormal, and at least the thresholdproportion of the predetermined number of selected events is abnormalfor at least a threshold number of events out of a preselected number ofevents. If the threshold proportion is increased, the method may alsoinclude increasing the threshold number and the preselected number.

Yet another illustrative embodiment includes a method of cardiactreatment using an implantable medical device system having implantedelectrodes and an energy storage system for storing charge forperforming cardiac electrical stimulus, the method comprising capturinga predetermined number of selected cardiac events using the implantedelectrodes, observing whether a cardiac condition exists for at leastone of the selected cardiac events, determining whether the cardiaccondition persists for at least a threshold duration or threshold numberof cardiac events, and, if so: initiating a charging operation for theenergy storage system, and determining whether cardiac condition hasterminated prior to delivery of cardiac stimulus and, if so, extendingthe threshold duration or increasing the threshold number.

Another illustrative embodiment includes a method of operating animplantable electrical device disposed for providing cardiac stimulus toa patient's heart, the method comprising observing electrical activityin a patient's thorax to discern cardiac function, using the discernedcardiac function to establish a metric related to the patient's cardiacfunction, determining whether treatment is indicated by comparing themetric to a threshold, preparing to deliver treatment by charging anenergy storage means coupled to the operational circuitry, observingwhether treatment continues to be indicated, and: if treatment continuesto be indicated, discharging energy from the energy storage means to thepatient, or, if treatment is no longer indicated, changing thethreshold. The metric may be related to a proportion of cardiac eventsthat are abnormal within a set of cardiac events, and/or the metric maybe related to a duration of time or a number of sensed cardiac events inwhich a malignant cardiac condition is detected.

Yet another illustrative embodiment includes a method of operation foran implantable medical device having an energy storage system forstoring and delivering therapeutic cardiac stimulus, the methodcomprising observing cardiac function using implanted electrodes bycapturing signals from the implanted electrodes and analyzing thecaptured signals using operational circuitry associated with theimplantable medical device, determining whether electrical cardiacstimulus is likely indicated using a first metric; if so, verifying thatelectrical cardiac stimulus is indicated using a second metric; if so,initiating a charging operation for the energy storage system; afterinitiating the charging operation, at least once checking that one ofthe first metric or the second metric continues to indicate electricalcardiac stimulus, and, if not, modifying a threshold used for comparisonwith either: the first metric in the determining step, and/or the secondmetric in the verifying step, to cause the threshold to become morerigorous. The checking step may be performed at least once during thecharging operation. The checking step may also be performed at leastonce after the charging operation is completed.

Another illustrative embodiment includes a method of operating animplantable medical device, the method comprising observing a firstthreshold to determine whether a patient in which the medical device isimplanted likely needs cardiac stimulus, and, if so, observing a secondthreshold to determine whether the cardiac stimulus is currentlyindicated, and, if so, initiating a charging sequence for an energystorage system configured for stimulus delivery to the patient; if not,returning to the step of observing the first threshold. The method mayinclude, if the charging sequence is initiated, performing the followingsteps if the charging sequence is completed: observing a third thresholdto determine whether cardiac stimulus is currently indicated; and if so,delivering cardiac stimulus to the patient using implanted electrodes;if not, observing the first threshold to determine whether the patientlikely needs cardiac stimulus. If the patient still likely needs cardiacstimulus, the method may include returning to the step of observing thethird threshold, or if the patient no longer likely needs cardiacstimulus, adjusting at least one of the first threshold and the secondthreshold. The method may further include observing whether the firstthreshold remains crossed during a time period between initiation andcompletion of the charging sequence and: if so, continuing the chargingsequence until completion; or, if not, stopping the charging sequenceand changing at least one of the first threshold and the secondthreshold.

Yet another illustrative embodiment includes a method of cardiactreatment using an implanted medical device system having implantedelectrodes and an energy storage system for storing electric charge foruse in cardiac electrical stimulus, the method comprising A) determiningwhether an abnormal event threshold is crossed, the abnormal eventthreshold being crossed if a specified number of abnormal events occurwithin a selected number of most recent detected cardiac events, and, ifso: determining whether the abnormal event threshold has been exceededfor a threshold number of most recent events and observing whether amost recent events is abnormal, and, if so, initiating a charge sequencefor the energy storage system. Once the charge sequence is complete, themethod may further include B) determining whether a most recent event isabnormal and, if so, delivering therapeutic energy to the patient; andif not, determining whether the abnormal event threshold remainsexceeded when measured from a most recent event; and: if so, waiting fora next cardiac event and again performing step B); or if not, raisingthe threshold number of most recent events and returning to step A). Themethod may further include determining whether the abnormal eventthreshold continues to be crossed while the charge sequence is beingperformed. Another illustrative embodiment includes a method ofoperating an implanted medical device system having implanted electrodesand an energy storage system for storing electric charge for use incardiac electrical stimulus, the method comprising observing a cardiacrate for a patient, and if the cardiac rate exceeds a predetermined ratethreshold, performing the above method.

Another illustrative embodiment includes a method of operation for animplanted medical device having implanted electrodes, an energy storagesystem for storing and delivering cardiac stimulus to a patient, andoperational circuitry for operating the device, the method comprisingcapturing a number of cardiac events, determining whether a firstcardiac event, along with a set of previously captured cardiac events,indicates a malignant cardiac condition and, if so, flagging the firstcardiac event, observing whether a threshold number of cardiac eventswithin a set of cardiac events have been flagged; if so, initiating acharging sequence for charging the energy storage system in preparationfor delivery of electrical cardiac stimulus. The illustrative methodfurther includes, after the charging sequence is initiated, at leastonce observing whether treatment of the patient continues to beindicated due to a malignant cardiac condition; and, if treatment is nolonger indicated, modifying the size of the threshold number and/or theset of cardiac events.

Another illustrative embodiment includes a method including implantingan implantable electrical device in the patient, wherein the device isconfigured to operate as noted in any the above illustrativeembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B illustrate, respectively, representative subcutaneous andtransvenous implantable cardiac treatment systems;

FIG. 2 shows an illustrative method for delivering therapy;

FIG. 3 depicts a charge initiation scheme in accordance with anillustrative embodiment of the present invention;

FIGS. 4A-4C depict heart rhythm sections where each individual cardiacevent is characterized;

FIG. 5 depicts a charge initiation sequence where an illustrativecardiac rhythm management device is unable to sense during charging;

FIG. 6 depicts a charge initiation sequence where an illustrativecardiac rhythm management device is able to sense during charging; and

FIG. 7 illustrates a dual testing approach for charging and therapydelivery decisions.

DETAILED DESCRIPTION

The following detailed description should be read with reference to thedrawings, in which like elements in different drawings are numberedidentically. The drawings, which are not necessarily to scale, depictselected embodiments and are not intended to limit the scope of theinvention. Those skilled in the art will recognize that many of theexamples provided have suitable alternatives that may be utilized.

The present invention is generally related to implantable cardiactreatment systems that provide therapy for patients who are experiencingparticular arrhythmias including, for example, ventricular tachycardia.The present invention is directed toward therapy delivery architecturesfor use in cardiac rhythm devices, as well as devices incorporating sucharchitectures. In particular, the present invention is suited forimplantable cardiac treatment systems capable of detecting and treatingharmful arrhythmias.

To date, implantable cardiac treatment systems have been eitherepicardial systems or transvenous systems such as the transvenous systemimplanted generally as shown in FIG. 1B. However, as further explainedherein, the present invention is also adapted to function with asubcutaneous implantable cardiac treatment system as shown in FIG. 1A.

FIG. 1A illustrates a subcutaneously placed implantable cardiactreatment system, in particular, an implantablecardioverter/defibrillator (ICD) system. In this illustrativeembodiment, the heart 10 is monitored using a canister 12 coupled to alead system 14. The canister 12 may include an electrode 16 thereon,while the lead system 14 connects to sensing electrodes 18, 20, and acoil electrode 22 that may serve as a shock or stimulus deliveryelectrode as well as a sensing electrode. The system may be implantedsubcutaneously as illustrated, for example, in U.S. Pat. Nos. 6,647,292and 6,721,597, the disclosures of which are both incorporated herein byreference. By subcutaneous placement, it is meant that electrodeplacement does not require insertion of an electrode into a heartchamber, in or on the heart muscle, or the patient's vasculature.

FIG. 1B illustrates a transvenous ICD system. The heart 30 is monitoredand treated by a system including a canister 32 coupled to a lead system34 including atrial electrodes 36 and ventricular electrodes 38. Anumber of configurations for the electrodes may be used, includingplacement within the heart, adherence to the heart, or dispositionwithin the patient's vasculature.

The present invention may be embodied by operational circuitry includingselect electrical components provided within the canister 2 (FIG. 1A) orcanister 11 (FIG. 1B). In such embodiments, the operational circuitrymay be configured to enable the methods to be performed. In some similarembodiments, the present invention may be embodied in readableinstruction sets such as a program encoded in machine or controllerreadable media, wherein the readable instruction sets are provided toenable the operational circuitry to perform the analysis discussed inthe described embodiments. Further embodiments may include a controlleror microcontroller adapted to read and execute the described methods.

The cardiac rhythm management device, whether subcutaneous ortransvenous, senses cardiac signals from the patient's heart. The mannerin which the data is collected and the type of data collected isdependent on the cardiac rhythm management device being used. Moreover,the cardiac rhythm management device may be programmed to, or mayautomatically adapt to, optimally detect a particular form of data whichis sought by the cardiac rhythm management device.

FIG. 2 shows an illustrative method for delivering therapy. As shown,the method includes a first step of observing cardiac function, as shownat 50. The method determines whether treatment is indicated, as shown at52. Treatment may be indicated by a number of different methods, forexample, by reliance upon any number of characteristics or features ofthe sensed cardiac function. For the illustrative method, treatment maybe indicated by the use of a defined threshold. The threshold utilizedmay take any number of forms depending upon the device's functionality.For example, the threshold may relate to cardiac event rates, cardiacevent intervals, lack of cardiac events (asystole or fibrillation),cardiac event morphology (QRS width being an example), or event-to-eventcorrelation.

If treatment is indicated, the method goes on to begin a chargingoperation, as shown at 54. The initiation of the charge is stored in anassociated energy system (sometimes a capacitor or bank of capacitors).During and/or after the charging operation is completed, the method thenincludes the step of determining whether treatment is still indicated,as shown at 56. If treatment is still indicated when charging iscomplete (and/or at intermediate times during charging, if so enabled)then the therapy is delivered as shown at 58. If step 56 is performedwhile charging is occurring, then charging may continue until chargingis completed. If treatment is no longer indicated when step 56 isperformed, then the method includes changing the threshold, as shown at60. In an illustrative embodiment, the threshold may be changed to makethe threshold more rigorous for satisfying. By making the threshold morerigorous, it is believed that it is less likely that a charge initiationwill commence (step 52) without ultimately delivering a therapy (step58).

The following illustrative example, encompassing FIGS. 3 and 4A-4C, withfurther illustration shown in FIGS. 5-6, is a relatively completeexample. It should be understood that the claims which follow reciteparts of the illustrative methods shown, and that not all steps oraspects of the illustrative example are necessary to the invention.Instead, the relatively complete example shown in FIGS. 3-6 is aidingthe understanding of those of skill in the art.

FIG. 3 depicts a charge initiation method in accordance with anillustrative embodiment of the present invention. The illustrativecardiac rhythm management device detects cardiac information, as shownat 70, which includes raw signal data from the patient. This raw signaldata may be preprocessed by the device, if so required or desired.Preprocessing steps may include smoothing of the detected data,differentiation, filtering and/or other preprocessing methodologiesknown in the art.

Detected data, whether preprocessed or raw, is then classified as beingeither a cardiac event (a heartbeat) or not a cardiac event (extraneousnoise). This classification may merely be an initial determination. Inparticular embodiments, sensed events may then be secondarily examinedin an additional waveform appraisal phase. The waveform appraisal phasefurther appraises the sensed cardiac events and substantiates theclassification of those detected signals as true cardiac events.Illustrative details of an example waveform appraisal method aredisclosed in detail in U.S. patent application Ser. No. 10/858,598,filed Jun. 1, 2004, now U.S. Pat. No. 7,248,921 and titled METHOD ANDDEVICES FOR PERFORMING CARDIAC WAVEFORM APPRAISAL, the disclosure ofwhich is incorporated herein by reference.

Whether from the raw or appraised cardiac data, the illustrative cardiacrhythm management device may then calculate a patient's representativecardiac rate, as shown at 72. In one embodiment, the cardiac rate iscalculated from the average R-R interval between four (4) consecutivesensed cardiac events. Other methods known in the art for calculating apatient's cardiac rate may also be utilized.

The illustrative cardiac rhythm management device then assesses whetherthe cardiac rate is tachyarrhythmic, as shown at 74. Cardiac rates inexcess of 100 bpm are defined as tachyarrhythmias in adults. For mostpatients, however, a higher cardiac rate threshold is more indicative ofa pathologic tachyarrhythmia. Some illustrative embodiments of theinvention account for this inconsistency with patients by permitting aphysician to adjust the cardiac rate threshold to meet the needs of aparticular patient. For example, a physician may adjust the cardiac ratethreshold to 120 bpm, instead of 100 bpm, to indicate tachyarrhythmia.This attribute is particularly beneficial for children, who generallyhave higher pathologic tachyarrhythmic thresholds (around 180 bpm) thanadults.

When a patient's cardiac rate is below the tachyarrhythmic threshold,the illustrative cardiac rhythm management device takes no furtheraction other than to continue monitoring the patient's cardiac rate.However, in instances where a patient's cardiac rate does exceed thethreshold, the illustrative device then further evaluates the individualcardiac events giving rise to the tachyarrhythmic rate. In particular,the illustrative device first employs an X out of Y counter, as shown at76.

The individual cardiac events giving rise to a tachyarrhythmic rate arehereinafter defined as “malignant cardiac events” and are the Xconstituent in the counter. The Y constituent for the counter comprisesthe total number of cardiac events being evaluated (whether malignant ornon-malignant). In a preferred embodiment, the cardiac rhythm managementdevice requires eighteen (18) malignant cardiac events out oftwenty-four (24) total detected events to prompt the cardiac rhythmmanagement device to initiate a second level of evaluation, as shown bythe decision at 78. Alternative X out of Y quotients may also be usedwith the present invention.

If the X out of Y counter requirement is not met, then the devicecontinues to monitor the patient's cardiac rate until the X out of Ycounter requirement is satisfied, if ever. Once the counter requirementis satisfied, the device evaluates the persistence of the malignantcardiac events, as shown at 80.

The persistence condition of the illustrative embodiment is agate-keeper step. As the gate-keeper, the persistence condition is thelast condition that must be satisfied prior to the illustrative deviceinitiates the charging of its capacitors for therapy delivery, as shownat 82. It is believed that the persistence condition will reduce thenumber of unnecessary device charge initiations. This is particularlytrue for those patients who suffer from non-sustained ventriculararrhythmias. Reducing inappropriate charges increases the longevity ofthe device's batteries and ultimately the longevity of the deviceitself. This subsequently benefits the patient by reducing the frequencyof device replacements.

The illustrative persistence condition evaluates a cardiac rhythm'spersistence in satisfying the X out of Y counter requirement.Specifically, the persistence condition evaluates two questions relatingto the X out of Y counter:

-   -   (1) Is the X out of Y counter requirement satisfied sequentially        for a defined ‘x’ number of times?    -   (2) Does the last cardiac event in the final X out of Y counter        requirement indicate a malignant cardiac event?

In an illustrative embodiment, the persistence condition is (at leastinitially) satisfied when the X out of Y counter requirement issatisfied sequentially for two (2) times. Examples illustrating thepersistence condition are depicted in FIGS. 4A-4C.

FIGS. 4A-4C depict heart rhythm sections where each individual cardiacevent is characterized. More specifically, the FIGS. depict a sequence90 of cardiac events where the “M” symbol represents a malignant cardiacevent and the “E” symbol represents a non-malignant cardiac event.

In FIG. 4A, nineteen (19) malignant events are shown in twenty-four (24)cardiac events. According to an 18 out of 24 counter requirement, thesection of cardiac events depicted satisfies the X out of Y counterrequirement. Since the counter requirement is satisfied, the device maythen evaluate the persistence condition. In doing so, the device flagsthis X out of Y counter as the first success for evaluating thepersistence condition.

FIG. 4B similarly depicts the sequence 90 of twenty-four (24) cardiacevents in FIG. 4A, but additionally includes a newly sensed twenty-fifth(25) cardiac event that is malignant, forming another sequence 92B oftwenty-four (24) cardiac events. With the addition of the new cardiacevent, the cardiac rhythm management device again evaluates the X out ofY counter requirement. Now, the last sequence 92B of twenty-four (24)cardiac events includes twenty (20) malignant events. Thus, the X out ofY counter requirement is again satisfied and the device flags thissuccessive X out of Y counter. The illustrative device's operationalcircuitry then evaluates the persistence condition. The first inquiry ofthe persistence condition is satisfied because two sequential X out of Ycounter requirements were met. Additionally, the second inquiry of thepersistence condition is satisfied because the last cardiac event in thesecond and final X out of Y counter requirement was a malignant event.As a result, the illustrative device's operational circuitry theninitiates the charging of the device's capacitors for therapy delivery.

FIG. 4C also depicts the sequence 90 of twenty-four (24) cardiac eventsin FIG. 4A, but instead includes a newly sensed twenty-fifth (25)cardiac event that is normal, or at least non-malignant, forming anothersequence 92C of twenty-four (24) cardiac events. When the cardiac rhythmmanagement device again evaluates the X out of Y counter requirement, itobserves that the last sequence 92B of twenty-four (24) cardiac eventsinclude eighteen (18) malignant events. Thus, the X out of Y counterrequirement is again satisfied and the device flags this successive Xout of Y counter. The illustrative device's operational circuitry thenevaluates the persistence condition. The first inquiry of thepersistence condition is satisfied because two sequential X out of Ycounter requirements were met. However, the second inquiry of thepersistence condition fails. Specifically, the last cardiac event in thesecond and final X out of Y counter requirement was not a malignantevent. Thus, the device refrains from charge initiation.

Once the gate-keeping function of the persistence condition issatisfied, the device begins to charge its capacitors. FIGS. 5 and 6depict two alternative scenarios for charging. FIG. 5 depicts a chargeinitiation sequence where an illustrative cardiac rhythm managementdevice is unable to sense during charging. More specifically, once thedevice initiates its charging cycle, as shown at 100, the device cannotsense, as shown during time period 102. The method resumes once thedevice charge is completed, as shown at 104. Since the illustrativecardiac rhythm management device cannot sense during charging, thedevice must wait until the capacitors are fully charged before theoperational circuitry may resume sensing. Immediately following chargecompletion 104, however, the device reconfirms that therapy delivery isnecessary as shown at 106.

In one embodiment, the reconfirmation process is deemed successful iftwo consecutive malignant events occur within six (6) contiguous cardiacevents, with the six (6) events being the most recent events availablefor evaluation. If the conditions set forth in the reconfirmationprocess 106 are satisfied, then the capacitors are again charged totheir fullest (if necessary) as shown at 108, and therapy is deliveredto the patient, as shown at 110.

Alternatively, if the reconfirmation process 106 fails, the device againemploys an X out of Y counter requirement, as shown at 112, and mustagain satisfy the persistence condition, as shown at 114. If at any timethe persistence condition and the reconfirmation process are satisfied,then the capacitors are again charged to their fullest, as shown at 108,and therapy is delivered to the patient, as shown at 110. However, ifthe patient converts to a normal sinus rhythm after charging (orotherwise fails the X out of Y counter requirement), then the parametersset for the persistence condition are modified, as shown at 116, and thecapacitors are bled of their charge, as shown at 118.

In addition to being a gate-keeper, the persistence condition is alsoadaptive to the particular needs of a patient. If a patient frequentlysuffers from non-sustained tachyarrhythmias, the persistence conditionsmay be adjusted to help eliminate premature charge initiations. In oneembodiment, the first inquiry is further defined to account for thisadaptability:

-   -   (1) Is the X out of Y counter requirement satisfied sequentially        for (3*n)+2 times, where n is an integer with limits between 0        and 5, and further wherein n starts at 0 and increases one        integer with each aborted charge initiation?        The second inquiry remains the same. With this expanded        definition, the persistence condition's first inquiry requires:    -   two (2) sequential X out of Y counter requirements satisfied if        the device has never aborted a charge initiation;    -   five (5) sequential X out of Y counter requirements satisfied if        the device has aborted a charge initiation once;    -   eight (8) sequential X out of Y counter requirements satisfied        if the device has aborted a charge initiation twice;    -   eleven (11) sequential X out of Y counter requirements satisfied        if the device has aborted a charge initiation three times;    -   fourteen (14) sequential X out of Y counter requirements        satisfied if the device has aborted a charge initiation four        times; and    -   seventeen (17) sequential X out of Y counter requirements        satisfied if the device has aborted a charge initiation five        times.        In all other aspects, the illustrative persistence condition        operates as described in detail above with reference to FIGS. 3        and 4A-4C.

FIG. 6 illustrates a portion of the charge initiation sequence where theoperational circuitry can sense cardiac events while the device ischarging. Cardiac events that are sensed during charging are evaluatedand assessed similarly to those being sensed when the device is notcharging or otherwise preparing to deliver therapy. Therefore, thecardiac rhythm management device may calculate a cardiac rate andevaluate the frequency of malignant events during the charging cycle. Assuch, after the device initiates its charging, as shown at 130, thedevice begins monitoring the X out of Y requirement, as shown at 132. Ifat any time during the device's charge cycle the patient converts to anormal sinus rhythm after charging (or otherwise fails the X out of Ycounter requirement), then the device stops charging, as shown at 134.Additionally, the capacitors are bled of their charge and the parametersset for the persistence condition are modified, as shown at 136, byapplying the expanded definition for the first inquiry described above.The method then returns to detection and rate calculation, as shown at138. Alternatively, if the patient remains in a tachyarrhythmic statethroughout the charge cycle, then the device reconfirms that therapydelivery is necessary through the reconfirmation process, following amethod similar to that shown in FIG. 5, as indicated at 140.

In the above illustrative embodiment shown in FIGS. 3-6, the term“malignant event” was defined as an event indicative of atachyarrhythmic rate. In other embodiments, any detected cardiac eventthat, upon analysis, is indicative of a malignant condition (i.e. acardiac condition requiring intervention) may be considered a malignantevent. An example of a malignant event satisfying other criteria may bean event or series of events having a morphology (for example, shape,QRS width, consecutive event correlation waveform analysis, etc.)indicative of a malignant arrhythmia. Some examples of analysis forproviding such determinations are shown in U.S. patent application Ser.No. 10/856,084, filed May 27, 2004, now U.S. Pat. No. 7,330,757 andtitled METHOD FOR DISCRIMINATING BETWEEN VENTRICULAR ANDSUPRAVENTRICULAR ARRHYTHMIAS, the disclosure of which is incorporatedherein by reference.

In alternative embodiments, several portions of the method may bemodified. For example, the X out of Y count requirement may be changedto use different parameters (instead of 18 out of 24, other ratios maybe used). In yet another embodiment, rather than an X out of Y counterrequirement, a group of cardiac events may be analyzed to determine agroup correlation waveform score. If the correlation is sufficientlylow, it may be determined that cardiac function is erratic to a degreeindicating a need for therapeutic delivery. In yet another modification,the persistence conditions themselves may also change. For example, thepersistence condition may require longer or shorter analytical periods,consideration of morphology, noise considerations, or the like.

In some embodiments, other conditions may be modified after a patienthas spontaneously recovered from a non-sustained tachyarrhythmia orother malignant condition. For instance, rather than altering thepersistence condition, the X of Y counter requirement may be changed.More specifically, if an illustrative method begins with an 18 out of 24counter requirement, the X out of Y counter requirement may extend to alarger number, higher percentage, or other requirement. For example, the18 out of 24 counter requirement may adjust to 20 out of 24 and/or 21out of 27. This change may be further incremented with each successivenon-sustained ventricular tachyarrhythmia. For example, after the firstaborted charge, the 18 out of 24 counter requirement becomes 21 out of27, which becomes 24 out of 30 on the second aborted charge and 27 outof 33 on the third aborted charge. In alternative illustrativeembodiments, more than one rule for a given persistence condition may bechanged at the same time.

Some embodiments may make use of a pure rate-driven shockingdetermination which does not make use of the X out of Y counterrequirement. For example, if the cardiac rate observed to initiate themethod shown in FIG. 3 (the query “tachyarrhythmic rate?”) exceeds apredetermined threshold, it may be determined that therapy isappropriate. Then, at a later time (e.g., during charging of theillustrative device's energy storage system or after charging iscomplete) the rate may be checked again. Re-checking the conditionsgiving rise to a decision to deliver therapy at a later time may revealthat stimulus is no longer indicated. In response, a system may raisethe threshold cardiac rate for shocking, or may extend a relevant timeperiod in which such a threshold cardiac rate must be held to initiatecharging.

FIG. 7 illustrates a dual testing approach for charging and therapydelivery decisions. The method begins by observing the patient's cardiacfunction, as shown at 200. The method then determines, as shown at 202,whether treatment is likely indicated. If treatment is not indicated,the method returns to observing cardiac function as shown at 200, wherethe method waits until a next cardiac event is sensed for evaluation.

If treatment is likely indicated at 202, the method next verifies thattreatment is needed, as shown at 204, using a different metric than thatused in step 202. One method of separating out these two steps is toobserve event intervals to determine whether treatment is likelyindicated (step 202) and, if so, to perform further analysis (possiblycombining noise and average interval analysis) to verify that the eventintervals do indeed indicate malignancy. Other first and second steps(202, 204) may be performed without deviating from the scope of theinvention.

Yet another example of a first and second tier for analysis is a systemwherein multiple vector views are available, for example as shown inU.S. patent application Ser. No. 10/901,258, filed Jul. 27, 2004, nowU.S. Pat. No. 7,392,085 and titled MULTIPLE ELECTRODE VECTORS FORIMPLANTABLE CARDIAC TREATMENT DEVICES, the disclosure of which isincorporated herein by reference. For example, a first sensing vectormay be used for the first determination that treatment is likelyindicated at 202. Additionally, a second, different sensing vector maythen be used to verify that treatment is indicated at 204.

If treatment is likely indicated at 202 and a need for treatment isverified at 204, the method next includes the step of initiating thecharging operation of the associated energy storage device or system(often a capacitor or group of capacitors), as shown at 206. The methodnext goes to a charge instruction block 208 which includes steps thatare performed if the device/system is able to perform sensing andanalysis operations during charging of the system's energy storagedevice. If sensing and analysis operations cannot be performed duringcharging, the method goes directly to step 214, which is explainedbelow.

In the charge instruction block 208, the method determines whether theenergy storage system is completed charging, as shown at 210. If not,the method includes observing whether treatment is still likelyindicated, as shown at 212. The metric for determining whether treatmentis still likely indicated in step 212 may be that of either of steps 202or 204, or may be a yet a different metric. If treatment is still likelyindicated, the method continues charging at 210 and iterates this loopuntil charging is complete. Once charging is complete, the methoddetermines whether a need for treatment can be verified again, as shownat step 214. As with step 212, a metric previously used in one of steps202, 204, or 212 may be used in step 214, or a new metric may be chosen.If the last verification step 214 is successful, then therapy isdelivered as shown at 216.

If verification step 214 fails, then the stored energy may benon-therapeutically discharged, as shown at 218, in any suitable manner.Alternatively, if desired, the capacitors may continue to hold chargefor a predetermined time period to assure that re-charging is notneeded. One suitable manner for discharge is to use the stored energy toperform diagnostic or maintenance type functions for the device or, ifthe device is used for pacing, to use the energy to provide pacingenergy.

Next, a threshold is adjusted, as shown at 220. Several thresholds maybe adjusted, but preferably adjustment is performed on a threshold usedin at least one of the preliminary steps 202, 204 that are used to makea determination that charging should be initiated at step 206. Thethreshold may be made more rigorous such that a decision to initiatecharging at 206 becomes more difficult. The method then returns toobserving cardiac function as shown at 200.

Returning to step 212, in the charge instruction block 208, if treatmentis no longer likely indicated at step 212, the method terminatescharging, as shown at 222, and discharges the energy storage in anysuitable manner, as shown at 224. The method then goes to step 220 andadjusts a threshold as before.

As used herein, the term metric may indicate any suitable measurement orresult from an analytical procedure. Example metrics include eventrates, X out of Y counter ratios, and correlation results. Furthermetrics include a time period in which a condition (such as a cardiactachyarrhythmia) persists or a how often an X out of Y counter isflagged.

A threshold may be rendered more rigorous in response to an abortedcharge initiation. Thus, at least ostensibly, subsequent tachyarrhythmicevents must be more sustainable to cross an adjusted threshold forinitiating the device's charging cycle. Examples of adjustablethresholds include metrics such as event rate, X out of Y counterratios, number of time intervals during which a malignant condition isdetected, and the number of X out of Y counter flags set.

Numerous characteristics and advantages of the invention covered by thisdocument have been set forth in the foregoing description. It will beunderstood, however, that this disclosure is, in many aspects, onlyillustrative. Changes may be made in details, particularly in matters ofshape, size and arrangement of parts without exceeding the scope of theinvention. The invention's scope is defined, of course, in the languagein which the claims are expressed.

1. A method of cardiac signal analysis in an implantable cardiacstimulus device comprising electrodes configured for implantation in apatient, the method comprising: capturing a signal from the electrodes;detecting, using analysis of the captured signal, a series of detectedevents; and determining whether a malignant arrhythmia appears likelyby: a) using an X/Y counter having an indication of likely malignantevents (X) out of a set of detected events (Y); and b) comparing the X/Ycounter to a predetermined counter ratio; and wherein the method alsocomprises the implantable device selecting the predetermined counterratio using the following rules: c) if nonsustained high rate eventconditions have not been previously detected, a first counter ratio isselected; and d) if nonsustained high-rate event conditions have beenpreviously identified, a second counter ratio is selected, wherein theselection of the second counter ratio remains in place for use inidentifying future malignant arrhythmias and avoiding unnecessarytreatment of future nonsustained high rate event conditions.
 2. Themethod of claim 1 wherein the second counter ratio reduces a likelihoodthat the device, in response to a nonsustained high rate eventcondition, will determine that a malignant arrhythmia appears likely,relative to the first counter ratio.
 3. The method of claim 2 furthercomprising, as a prerequisite to performing the step of determiningwhether a malignant arrhythmia appears likely, identifying a likelihoodof present arrhythmia from the detected events such that, if nolikelihood of present arrhythmia is identified, the step of determiningwhether a malignant arrhythmia appears likely is not performed followingat least one detected event.
 4. The method of claim 3 wherein the devicefurther comprises a stimulus capacitor and associated charging circuitryand, if a malignant arrhythmia appears likely, the method furthercomprises the following: preparing to deliver therapy by charging of thestimulus capacitor; and continuing to observe cardiac function whilepreparing to deliver therapy and, if malignant arrhythmia continues toappear likely, delivering therapy to the patient once the device isprepared to deliver the therapy.
 5. The method of claim 4 wherein theelectrodes are all implanted subcutaneously in the patient, without anyelectrode disposed in or on the patient's heart.
 6. The method of claim4 wherein at least one electrode is transvenously implanted.
 7. Themethod of claim 3 wherein the step of identifying a likelihood ofpresent arrhythmia includes calculating an event rate across a series ofdetected events and determining whether the event rate exceeds athreshold.
 8. The method of claim 1 wherein the device further comprisesa stimulus capacitor and associated charging circuitry and if amalignant arrhythmia appears likely, the method further comprises thefollowing: preparing to deliver therapy by charging of the stimuluscapacitor; and continuing to observe cardiac function while preparing todeliver therapy and, if malignant arrhythmia continues to appear likely,delivering therapy to the patient once the device is prepared to deliverthe therapy.
 9. A method of cardiac signal analysis in an implantablecardiac stimulus device, the device comprising electrodes configured forimplantation in a patient and telemetry circuitry allowing an operatorto program parameters used in the device to determine whether to delivertherapy, the method comprising: capturing a signal from the electrodes;detecting, by analysis of the signal, detected events in the signal; anddetermining whether a malignant arrhythmia appears likely by: a) usingan X/Y counter having an indication of likely malignant events (X) outof a set of detected events (Y); b) comparing the X/Y counter to apredetermined counter ratio; and c) if the X/Y counter exceeds thepredetermined ratio, determining whether the X/Y counter has exceededthe predetermined ratio at least N times and, if so, determining that amalignant arrhythmia appears likely; wherein N is automaticallypredetermined by the implantable device such that N has an initial valueand, following identification of nonsustained apparently malignantconditions, N is increased to a higher value than the initial value andremains at the higher value for use in subsequent event processing toidentify future malignant arrhythmias and avoid treatment for futurenonsustained high-rate condtiions.
 10. The method of claim 9, furthercomprising identifying a likelihood of present tachyarrhythmia from thedetected events using event rate.
 11. The method of claim 10 wherein thedevice further comprises a stimulus capacitor and associated chargingcircuitry and, if a malignant arrhythmia appears likely, the methodfurther comprises the following: preparing to delivery therapy bycharging the stimulus capacitor; and continuing to observe cardiacfunction while preparing to deliver therapy and, if malignant arrhythmiacontinues to appear likely, delivering therapy to the patient once thedevice is prepared to deliver the therapy.
 12. The method of claim 11wherein the electrodes are all implanted subcutaneously in the patient,without any electrode disposed in or on the patient's heart.
 13. Themethod of claim 11 wherein at least one electrode is implantedtransvenously.
 14. The method of claim 10 wherein the step ofidentifying a likelihood of present arrhythmia includes calculating anevent rate across a series of detected events and determining whetherthe event rate indicates a tachyarrhythmia.
 15. The method of claim 10wherein the step of identifying a likelihood of present arrhythmiaincludes calculating an event rate across a series of detected eventsand determining whether the event rate exceeds a threshold.
 16. Themethod of claim 9 wherein the device further comprises a stimuluscapacitor and associated charging circuitry and, if a malignantarrhythmia appears likely, the method further comprises the following:preparing to deliver therapy by charging the stimulus capacitor; andcontinuing to observe cardiac function while preparing to delivertherapy and, if malignant arrhythmia continues to appear likely,delivering therapy to the patient once the device is prepared to deliverthe therapy.
 17. An implantable cardiac stimulus device comprising: acanister containing a battery, charging circuitry, energy storagecircuitry, control circuitry, and coupling circuitry, in which thecontrol circuitry controls operation and use of the charger, energystorage and coupling circuitry to provide cardiac signal monitoring and,when indicated, cardiac therapy; and a plurality of electrodes connectedto the coupling circuitry which, in turn, couples the control circuitryto the plurality of electrodes to allow cardiac signal monitoring and,when indicated, cardiac therapy; wherein the control circuitry isconfigured to perform the following method: capturing cardiac signalsfrom the electrodes via the coupling circuitry when implanted in apatient to detect the patient's cardiac condition; determining fromcaptured signals whether a likely arrhythmia is occurring; and, if alikely arrhythmia is occurring: determining whether the device haspreviously identified indication(s) of nonsustained tachyarrhythmia(s);selecting between a first X/Y counter ratio and a second X/Y counterratio for use in determining whether cardiac stimulus is indicated byusing at least a step of determining whether previous nonsustainedtachyarrhythmia(s) have occurred; applying the selected X/Y counterratio to determine whether cardiac stimulus is indicated; furtherwherein, if previous nonsustained tachyarrhythmia(s) have been detected,the selection between the first and second X/Y counter ratios reducesthe likelihood that a new nonsustained tachyarrythmia could lead totherapy delivery by the device.
 18. The implantable cardiac stimulusdevice of claim 17 wherein the control circuitry is further configuredsuch that: if previous nonsustained tachyarrhythmia(s) have beendetected, the second X/Y counter ratio is selected, else the first X/Ycounter ratio is selected; and the second X/Y counter ratio uses alarger X and larger Y than the first X/Y counter ratio.
 19. Theimplantable cardiac stimulus device of claim 17 wherein the controlcircuitry is further configured such that: if previous nonsustainedtachyarrhythmia(s) have been detected, the second X/Y counter ratio isselected, else the first X/Y counter ratio is selected; and the secondX/Y counter ratio requires consideration of a larger sampling of cardiacsignals than the first X/Y counter ratio.
 20. The implantable cardiacstimulus device of claim 17 wherein the control circuitry is configuredsuch that, if cardiac stimulus is indicated, the method furthercomprises the following: preparing to deliver therapy by charging theenergy storage circuitry; and continuing to observe cardiac functionwhile charging the energy storage circuitry and, if cardiac stimuluscontinues to be indicated, delivering therapy to the patient once thedevice is prepared to deliver the therapy.
 21. The implantable cardiacstimulus device of claim 17 wherein the first counter ratio is 18/24 andthe second counter ratio is 21/27.
 22. An implantable cardiac stimulusdevice configured for implantation into a patient's body comprising ahousing, operational circuitry contained in the housing, and a number ofelectrodes coupled to the operational circuitry and disposed either onthe housing, as part of the housing, or on one or more leads coupled tothe housing, wherein the operational circuitry is configured to performa method of cardiac signal analysis comprising: capturing signals fromthe electrodes when implanted in a patient; identifying detected eventsin the captured signals with the intent that detected events representcardiac events; determining whether a high-rate arrhythmia appearslikely; if so, analyzing detected events to identify those which likelyindicate a malignant cardiac rhythm; and determining whether an X/Ycounter in which X represents the number of detected events identifiedas likely indicating a malignant cardiac rhythm out of a set of Ydetected events indicates therapy for the patient; wherein theoperational circuitry is configured to identify nonsustained tachycardiaand, if nonsustained tachycardia is identified, to determine whether tomodify the X/Y counter to use a larger sample size and the larger samplesize is used for future cardiac signal analysis to avoid therapy onfuture nonsustained tachycardia.
 23. An implantable cardiac stimulusdevice comprising an implantable canister and a plurality of electrodes,the canister containing operational circuitry configured to performcardiac signal captured and analysis, as well as defibrillationcircuitry configured to provide defibrillation therapy if indicated byanalysis of cardiac signal, the operational circuitry further configuredto perform a method of cardiac signal analysis comprising: capturing asignal from the electrodes when implanted in a patient; detecting, usinganalysis of the captured signal, a series of detected events; anddetermining whether a malignant arrhythmia appears likely by: a) usingan X/Y counter having an indication of likely malignant events (X) outof a set of detected events (Y); b) comparing the X/Y counter to apredetermined counter ratio; and c) if the X/Y counter exceeds thepredetermined counter ratio, determining whether the predeterminedcounter ratio is exceeded by the X/Y counter for at least N iterationsand, if so, determining that a malignant arrhythmia appears likely; andwherein the method also comprises the implantable device selecting N asfollows: if nonsustained high rate event conditions have not beenpreviously detected, a first number is selected; and if nonsustainedhigh-rate event conditions have been previously identified, a secondnumber larger than the first number is selected; wherein the selectionof the second number remains in place for use in identifying futuremalignant rhythms.
 24. The implantable cardiac stimulus device of claim23 wherein the operational circuitry is further configured such thatreliance upon the second number reduces a likelihood that the device, inresponse to a nonsustained high rate event condition, will determinethat a malignant arrhythmia appears likely, relative to the firstnumber.